Field of the Invention
The invention relates to a transistor configuration for bandgap circuits.
Bandgap circuits are used so that precise reference voltages can be provided. Precise reference voltages are needed in many fields of technology, for example, in analog evaluation circuits and in the field of digital circuits. The provision of a precise reference voltage is also required in the field of semiconductor memory components, for example, in DRAM memory components, so that a precise and fast functionality of the DRAM memory component can be guaranteed.
Bandgap circuits are described, for example, in Tietze and Schenk, 9th edition, Springer-Verlag, Ch. 18.4.2 (ISBN 3-540-19475-4). In bandgap circuits, the reference voltage is fixed, for example, by a series circuit of a resistor and a transistor. The resistor has a positive temperature coefficient. The temperature coefficient of the base-emitter voltage of the transistor, conversely, is negative. The resistor and the transistor are chosen in such a way that the temperature coefficients have the same magnitude, but different signs. For a fixed reference voltage, which is fixed by the material of the transistor, the temperature coefficient of the reference voltage is xe2x80x9c0xe2x80x9d. The reference voltage is therefore independent of the temperature.
German Published, Non-Prosecuted Patent Application DE 4111 103 A1, corresponding to U.S. Pat. No. 5,229,710, discloses a CMOS bandgap reference circuit, with which a substantially temperature-independent reference voltage is generated. In this case, the bandgap circuit uses a parasitic bipolar transistor. An n-conductive well is provided in a positively conductive semiconductor substrate. The well contains more heavily doped zones with positive conductivity and more heavily doped zones with negative conductivity. Such a configuration gives rise to a parasitic vertical pnp bipolar transistor, whose p-type substrate acts as a collector. The n-type well with the heavily doped zones acts as a base and the heavily positively doped diffusion zone acts as an emitter. The construction of the bandgap reference circuit is made possible by parallel connection of two parasitic bipolar transistors and by the use of an operational amplifier and further resistors. The base-emitter voltage of the first parasitic transistor has a negative temperature coefficient. The difference between the base-emitter voltages between the first and the second parasitic transistors, conversely, has a positive temperature coefficient. The base-emitter voltage of the first transistor and the difference of the base-emitter voltages of the transistors are used as input signals for the operational amplifier. The negative and positive temperature coefficients are balanced through a suitable choice of the resistances, so that there is no dependency of the reference voltage on the temperature in a reference circuit, which corresponds to the bandgap of silicon.
It is accordingly an object of the invention to provide a transistor configuration for use in a bandgap circuit that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that has the advantage that a correspondingly configured structure with a first, second and third doped region, which is surrounded by a semiconductor substrate, is formed in semiconductor technology, especially for the production of DRAM memory components. The existing structure can be used according to the teachings of the invention to form a parasitic transistor for a bandgap circuit. It is therefore unnecessary to specially produce the embodiment known from the prior art, but rather the already existing structure can be used to produce the transistor.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a transistor configuration for a bandgap circuit containing a substrate and a bipolar transistor formed in the substrate. The transistor has first, second and third doped regions, the first doped region embedded in the second doped region, the second doped region surrounded by the third region, the third region separating the second region from the substrate and surrounded by the substrate, and the first and third regions doped opposite to the second region and the substrate.
In accordance with another feature of the invention, the first and third regions are negatively doped and the second region and the substrate are positively doped. In this way, a structure that exists in many embodiments of semiconductor components can be used to form a transistor for a bandgap circuit.
In accordance with a further feature of the invention, the first region is heavily doped and the second and third regions are weakly doped in given zones. A transistor with good electrical properties is made possible by corresponding choice of the doping.
In accordance with a further feature of the invention, there is provided a fourth region more heavily doped than the third region, having the same polarity as the third region and introduced into the third region as a first terminal. A fifth region more heavily doped than the second region, having the same polarity as the second region and introduced into the second region as a second terminal is also provided. A heavily doped sixth region, having the same polarity as the first region and introduced into the first region as a third terminal is also included.
In accordance with a further feature of the invention, heavily doped regions are used to make contact with weakly doped regions. In this way, a smaller ohmic contact is produced between the electrical line and the weakly doped regions.
In accordance with a further feature of the invention, there is provided a resistor having first and second terminals. The transistor is connected in series with the second terminal of the resistor and is connected to ground as a diode circuit. A reference voltage is tapped from the first terminal of the resistor.
In accordance with a further feature of the invention, the series connection of the transistor with the resistor forms one of two current paths of a bandgap circuit. Another transistor and a second resistor form another of the two current paths; the other transistor is connected to ground.
In accordance with a further feature of the invention, the bandgap circuit has a further current path formed in parallel with the two current paths; the further current path has a further transistor configured according to the transistor and the further transistor is connected to ground.
In accordance with a further feature of the invention, the transistor according to the invention is connected in series with a resistor in a first current path. In this case, the transistor is connected to ground. The reference voltage, which is made temperature-compensated by the bandgap circuit, can be tapped from the input of the resistor. The chosen configuration provides a straightforward embodiment of the bandgap circuit.
In accordance with a further feature of the invention, further simplification of the bandgap circuit is achieved if a second current path is formed from a series circuit of a transistor and a resistor. The second current path is a part of a current mirror via which the current flowing on the second current path is also imposed in the first current path. Very precise tuning of the temperature coefficient is possible by virtue of the symmetrical construction of the two current paths.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a transistor configuration for a bandgap circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.